(1) Field of the Invention
The present invention generally relates to electrochemical sensors and more particularly to reference half-cells for use in pH, oxidation/reduction potential, and selective ion activity measurements.
(2) Background Information
Electrochemical potential measurements are commonly used to determine solution pH, other selective ion activities, ratios of oxidation and reduction activities, as well as other solution characteristics. A pH/ion selective electrode/oxidation reduction potential meter (hereafter referred to as a pH/ISE/ORP meter) is typically a modified voltmeter that measures the electrochemical potential between a reference half-cell (of known potential) and a measuring half-cell. These half-cells, in combination, form a cell, the electromotive force (emf) of which is equal to the algebraic sum of the potentials of the two half-cells. The meter is used to measure the total voltage across the two half-cells. The potential of the measuring half-cell is then determined by subtracting the known potential of the reference half-cell from the total voltage value.
The measuring half-cell typically includes an ion selective material such as glass. The potential across the ion selective material is well known by those of ordinary skill in the art to vary in a manner that may generally be described by the Nernst Equation, which expresses the electrochemical potential as a logarithmic function of ion activity (thermodynamically corrected concentration). A pH meter is one example of a pH/ISE/ORP meter wherein the activity of hydrogen ions is measured. pH is defined as the negative logarithm of the hydrogen ion activity and is typically proportional to the measured electrochemical potential.
FIG. 1 is a schematic of a typical, prior art arrangement 20 for measuring electrochemical potential. Arrangement 20 typically includes a measuring half-cell 30 and a reference half-cell 40 immersed in a process solution 24 and connected to an electrometer 50 by connectors 38 and 48, respectively. Measuring half-cell 30 and reference half-cell 40 are often referred to commercially (as well as in the vernacular) as measuring electrodes and reference electrodes, respectively. Electrometer 50 functions similarly to a standard voltage meter in that it measures a D.C. voltage (electrochemical potential) between measuring half-cell 30 and reference half-cell 40. Measuring half-cell 30 typically includes a half-cell electrode 36 immersed in a half-cell electrolyte 32, which is typically a standard solution (e.g., in pH measurements). For some applications, such as pH measurement, measuring half-cell 30 also includes an ion selective material 34. Alternately, when measuring ORP the half-cell electrode 36 is immersed directly into the process solution 24.
The purpose of the reference half-cell 40 is generally to provide a stable, constant (known) potential against which the measuring half-cell may be compared. Reference half-cell 40 typically includes a half-cell electrode 46 immersed in a half-cell electrolyte 42 (FIG. 1). As used herein, the term xe2x80x9chalf-cell electrodexe2x80x9d shall refer to the solid-phase, electron-conducting material in contact with the half-cell electrolyte, at which contact the oxidation-reduction reaction occurs that establishes an electrochemical potential. Half-cell electrolyte 42 (FIG. 1) is hereafter referred to as a reference electrolyte. Electrochemical contact between the reference electrolyte 42 (FIG. 1) and the process solution is typically established through a reference junction 44, which often includes a porous ceramic plug or the like (e.g., porous Teflon(copyright), porous kynar(copyright), or wood) for achieving restricted fluid contact. Ideally, the reference junction 44 is sufficiently porous to allow a low resistance contact (which is important for accurate potential measurement) but not so porous that the solutions become mutually contaminated.
However, for many applications, particularly those having a relatively high ion concentration and/or those at a relatively high temperature, ion contamination is a significant difficulty. Both contamination of the reference electrolyte with process solution components and contamination of the process solution with reference electrolyte components are relatively common. Further, clogging of the reference junction with a variety of contaminants (e.g., process solution salts or silver chloride from the reference electrolyte) is also a relatively common difficulty with typical commercial reference electrodes. Both ion contamination and reference junction clogging may lead to unstable and/or erroneous measurements and therefore tend to be undesirable and problematic.
Turning now to the known art, there are several attempts to overcome the above stated difficulties. For example, U.S. Pat. No. 4,495,052 to Brezinski and U.S. Pat. No. 4,495,053 to Souza (hereafter referred to as the ""052 and ""053 patents, respectively) disclose reference electrodes having a removable and replaceable reference junction, the reference junction typically consisting of a ceramic plug within a glass tube. The ""052 and ""053 patents, while possibly providing for improved convenience, do not provide an ion-barrier and therefore do not tend to reduce ion contamination. The reference junctions disclosed therein may also be fragile and prone to breakage during removal and insertion.
Brezinski, in U.S. Pat. No. 4,401,548 (hereafter referred to as the ""548 patent) and in Analytica Chemica Acta, 134 (1982) 247-262, discloses a double junction type reference electrode having an internal diffusion barrier between the reference electrolyte and the junction electrolyte. The diffusion barrier consists of a porous glass rod (e.g., Vycor(copyright), manufactured by Corning Glass Works, Corning, N.Y.). Because these reference junctions are porous, their effectiveness tends to be limited, especially at elevated temperatures where the diffusion velocity of ions increases greatly. Further, these reference junctions are disposed in an internal cavity within the reference electrode, which tends to substantially complicate replacement. As such, failure of the reference junction may result in the need to replace the entire reference electrode.
Nipkow, et al., in U.S. Pat. No. 5,470,453 (hereafter referred to as the ""453 patent) disclose a double junction type silver/silver chloride reference electrode that features a silver ion reducing agent acting as a silver ion-barrier layer to reduce contamination of the junction electrolyte and reference junction with silver ions and/or silver chloride precipitate. As described above with respect to the ""548 patent, these reference junctions are disposed in an internal cavity within the reference electrode and, therefore, tend to be difficult to replace. Further, the reference junction disclosed in the ""453 patent is not configured to eliminate migration of process solution components (e.g., ions or other mobile species) into the reference electrolyte. Contamination of the reference electrolyte may therefore be problematic in some applications.
Therefore, there exists a need for an improved reference electrode and/or reference electrode junction for use in pH, selective ion activity, oxidation-reduction potential (ORP), and other electrochemical potential measurements that overcomes the aforementioned difficulties.
In one aspect, the present invention includes a modular reference junction for a reference half-cell. The reference junction includes a body including a reference electrolyte interface portion, a process solution interface portion, and an internal cavity disposed therebetween. The reference junction further includes an ion-barrier membrane disposed at said reference electrolyte interface portion and is sized and shaped for removable receipt within a receptacle of a reference half-cell housing. In one variation of this aspect, the reference junction includes an ion-barrier membrane shaped substantially in the form of a cylindrical tube and including poly(perfluorosulfonic acid).
In another aspect, this invention includes a reference half-cell. The reference half-cell includes a half-cell electrode, a reference electrolyte, and a reference junction positioned in an outlet for the reference electrolyte, the reference junction being sized and shaped for removable receipt within the outlet. The reference junction includes a junction electrolyte and an ion-barrier membrane disposed between the junction electrolyte and the reference electrolyte. In one variation of this aspect the half-cell electrode includes silverxe2x80x94silver chloride, the reference electrolyte is an aqueous solution including a mixture of potassium chloride and silver chloride, and the ion-barrier membrane includes poly(perfluorosulfonic acid).
In still another aspect, this invention includes an electrochemical potential measurement sensor. The sensor includes a measuring half-cell and a reference half-cell including a half-cell electrode, a reference electrolyte, and a reference junction positioned in an outlet for the reference electrolyte, the reference junction being sized and shaped for removable receipt within the outlet. The reference junction includes a junction electrolyte and an ion-barrier membrane disposed between the junction electrolyte and the reference electrolyte. In one variation of this aspect the measuring half-cell and reference half-cell are mounted in a common housing.
In yet another aspect, this invention includes a method for measuring electrochemical potential. The method includes providing a reference half-cell including a half-cell electrode, a reference electrolyte, and a reference junction positioned in an outlet for the reference electrolyte, the reference junction being configured for selectively inserting and removing from the outlet, the reference junction further including a junction electrolyte and an ion-barrier membrane disposed between the junction electrolyte and the reference electrolyte. The method further includes providing a measuring half-cell, inserting the reference half-cell and the measuring half-cell in a liquid; and electrically connecting the reference half-cell and the measuring half-cell to a voltage meter.
In a further aspect this invention includes a method for fabricating a reference junction for a reference half-cell. The method includes providing a body including a reference electrolyte interface portion, a process solution interface portion, and an internal cavity disposed therebetween, providing an ion-barrier membrane, disposing said ion-barrier membrane at the reference electrolyte interface portion, and sizing and shaping the reference junction for removable receipt within a receptacle of a reference half-cell housing.